Why Do We Need the Priesthood on the Earth?

We must have priesthood authority to act in the name of God when performing the sacred ordinances of the gospel, such as baptism, confirmation, administration of the sacrament, and temple marriage. If a man does not have the priesthood, even though he may be sincere, the Lord will not recognize ordinances he performs (see Matthew 7:21–23; Articles of Faith 1:5). These important ordinances must be performed on the earth by men holding the priesthood.

Men need the priesthood to preside in The Church of Jesus Christ of Latter-day Saints and to direct the work of the Church in all parts of the world. When Christ lived on the earth, He chose His Apostles and ordained them so that they could lead His Church. He gave them the power and authority of the priesthood to act in His name. (See Mark 3:13–15; John 15:16.)
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Another reason the priesthood is needed on the earth is so we can understand the will of the Lord and carry out His purposes. God reveals His will to His authorized priesthood representative on the earth, the prophet. The prophet, who is President of the Church, serves as the spokesman for God to all members of the Church and all people on the earth. - By LDS.ORG